Sensory Analysis and Shelf-Life Testing

Lynn A. Kuntz, 1993

If there is one thing everyone agrees on regarding the use of sensory analysis in shelf life testing, it's that sensory is one of the best tools available. From there, the paths of the experts may diverge in the search to implement the best program. However, ultimately they all reach the same place: assurance that the products tested will meet a high level of consumer acceptance for a specified period of time.

Begin Communications

Before the paths diverge, the experts all concur on the place to start. Communication between sensory, product development, analytical, marketing, packaging and other pertinent groups is crucial.

"The more information sensory has about a product and the purpose of a project, the better they can design a test which answers the correct questions in the most efficient manner," states Rebecca Newby, manager of sensory projects, Tragon Corp., Redwood City, CA. It is important to know the objectives of the project -- whether it's the development of a new product or process, seeking an alternate ingredient or determining the best packaging material."

Follow Procedures

"Almost all forms of sensory evaluation methods can be used in shelf life evaluation," says Gail Vance Civille, president, Sensory Spectrum, Chatam, NJ. Specifically, the appropriate methods include:

-- Discrimination Tests. These can be performed by untrained panelists or by those specifically screened for an ability to detect a specific difference. "We use people who are sensitive to off-flavors and have good color perception," notes David Ash, vice president of research and development, Del Monte Foods, Walnut Creek, CA. "They can identify things that become the limiting factor in a product's shelf life. No one is on a panel unless they like that type of product."

These tests are used when it is obvious what product characteristic has changed, since the method gives no information regarding the type of difference.

Difference panels require approximately 30 to 50 panelists. According to Dr. Susan Cuppett, associate professor at the University of Nebraska's Department of Food Science, Lincoln, NE, "too high a number of panelists can bias as well as too low. If you have more than a hundred people, statistically the numbers begin to skew one way or another. It's a misconception that the higher the numbers, the better the results."

-- Duo/Trio tests use three samples, one of which is different. One of the matching samples is labeled as the reference, and the panelist is asked which of the two other samples matches the reference sample.

-- Triangle tests use three samples, one of which is different. The panelist must determine which one of the three samples is the different one. This test enjoys a slight advantage over the duo/trio as it requires a smaller number of tests to achieve statistical significance.

-- Descriptive Tests. Quantitative descriptive analysis (QDA) or flavor profile analysis (FPA) must be performed by a trained panel so that the terms mean the same thing to each of the panelists. The training can be costly in terms of time -- an expert panel has to be trained for a specific type of product.

"If you require detailed information -- what attributes have changed with time and how -- descriptive techniques are extremely valuable," says Cuppett.

Panel sizes are small, typically around ten. Cuppett recommends training one or two additional people as alternates in cause they're needed.

"Sometimes panelists get off track and must be retrained. If they are unable to retrain, you might drop their evaluations and go to an alternate."

-- Affective tests. Hedonic scales -- which indicate acceptance on a nine point numerical scale labeled from "dislike extremely" to "like extremely" are typically used for shelf-life evaluations. These tests can be worded to not only show the overall acceptance of the product, but that of specific characteristics such as flavor and texture. Trained panels can also use this technique on a line scale which can be converted to numerical equivalents.

Paneling the tests

The evaluation technique may vary, depending on the resources available and the information needed. The panelist contingent can vary according to the requirements of the test and the evaluation itself: experts, employees, consumers. However, another point of agreement shows up regarding the panelists.

"We make a point not to have project people on the panel to limit the bias," states Ash. Newby agrees, "People are biased when they know something about the product; the only way to remove the bias is to test on a blind basis. But you can use a pragmatic approach -- sensory testing is expensive. Tabletop panels can be used for screening and if the product is obviously unacceptable you can avoid unnecessary testing."

Using your control

There are various camps when it comes to using a control product. Some sensory experts prefer a physical control; others are satisfied to just use the numbers obtained in the zero time evaluation. According to Civille, there are three alternatives when using a physical sample as a control.

-- Making the control from scratch each time using the same ingredients, procedures, etc. if it's a simple mix.

-- Freezing the control and accepting that it might have changed slightly, but minimally compared to the product in shelf life.

-- Using a fresh batch of product which may not be identical.

Civille believes that one of the problems with doing a shelf life without a control is that "consumers may be rating it in the context of a similar product on their shelf which may not be fresh either. They may not be as critical as if there was a control to evaluate."

Ash suggests that the best conditions for preserving control product is to hold frozen product at a constant -10 degrees F temperature and shelf-stable products at 35 degrees F. This limits any product changes due to phase changes, such as water or fat crystallization, and in general, retards product deterioration.

The age of the control or zero-time product for evaluation varies with the product. The product may be freshly prepared or manufactured. However, the persons with the responsibility of defining the product concept may choose to set the "zero-time" evaluation at the time at which a product reaches equilibrium. This is possible with a multiphase product subject to water, oil or flavor migration. The zero-time can also be established at the time the product typically reaches the consumer to avoid testing against a product, that in reality, the consumer will never see.

Moving and Storage

The storage conditions for an ambient shelf-life study should mimic those it will encounter in real life. That includes not only temperature and freeze/thaw cycles, but also factors such as humidity, light, packaging and handling.

"It's impossible to mimic every condition the product will ever encounter," notes Peter Putnam, sensory analyst, McCormick & Company, Inc., Hunt Valley, MD. "Therefore, it's important to test the range of conditions they're likely to encounter. Then anytime the product actually falls into that set of conditions, you can be fairly certain that it won't fail."

Deciding what the proper storage conditions entail should be the responsibility of the product development person or team, not just the sensory analyst, offers Civille. Accelerated conditions may be helpful, if there is a good correlation between shelf-life curves under those conditions and those under ambient conditions for similar products.

"But don't assume that every line extension will have the same shelf life as the original," warns Civille.

Another point to which everyone readily agrees is to store more product than you think you'll need for the study. Estimates range from 25 to 50% more. This is to provide product for retest or other emergencies, and to provide a pool of product to insure random selection. Make certain that each single unit put into storage is properly labeled to avoid sample mix-ups.

Finding the End

When a product is no longer considered acceptable, it has reached it's endpoint. But where is that point set?

"There's no government regulation defining the product endpoint," explains Newby. "There's a microbiological shelf life, but you must be much more stringent than that. At what point does consumer acceptance decrease or does the product change so that there is a noticeable difference? These are dependent on your corporate objectives and how much risk the company is willing to take with the brand."

Rarely does a company use the criterion of no detectable change. Often the endpoint is established at a certain number or a specific drop in consumer acceptability. The product should fail when it no longer represents the product concept.

The Replacements

Because of the scope of product development at most companies, the sensory department could devote 110% of its time to shelf-life studies. To relieve the burden of continuous shelf-life studies, there are a few remedies. If the shelf life can be estimated with any accuracy, the test intervals can be lengthened and clustered around the expected failure period. Most of the experts only require about six evaluations to provide reliable results. In some cases, four may be adequate. Monthly evaluation of a product with an estimated shelf life of one year can be overkill unless there is a valid reason for closely studying the changes in product characteristics, such as the establishment of an accelerated shelf life curve.

Another way to cut the through the sensory/shelf-life log jam is to rely on chemical or instrumental analyses that closely relate to sensory analysis. Moisture, free fatty acids or color measurement can supplement the sensory techniques.

"If you have developed a relationship between an instrumental method and the shelf life of a product, use it!" urges Newby. "They're usually less expensive and time consuming than sensory."

Putnam notes that a correlation between a physical or chemical test can increase the confidence level of the sensory results. Most sensory experts agree that analytical methods should complement the sensory tests. If not used as a complete substitution, the baseline and endpoint evaluations should be confirmed by sensory techniques. Still, analytical techniques often are overlooked.

"Things that are not sensory attributes are often overlooked," observes Putnam. "Functionality of the product or the packaging may not be examined if shelf life is considered solely a sensory function."

For example, if the whipped topping mix still has favorable sensory scores, but only whips to half of its expected volume, it may have reached a functional endpoint rather than a sensory one.

Used correctly, sensory can measure how a product changes over time.

"It's better not to conduct a shelf life study than to conduct a poorly designed one. You can spend a lot of time and resources and come up with nothing, or worse yet, the wrong answer," says Putnam.

And that can result in more than wasted time, it may produce a marketplace embarrassment. So, make sure you're headed down the right path.

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